A large number of various apparatuses are known for examining or testing elongated textile test subjects such as card slivers, roving, yarns or fabrics for example. Depending on their application, they can be categorized into the two classes of laboratory test (offline) and testing during the production process (online). The apparatuses make use of the various known sensor principles, of which the capacitive measuring principle is of special interest in this case, wherein a measuring capacitor is typically arranged as a planar plate capacitor and comprises a through-opening for the test subject. The measuring capacitor is part of an LC oscillator, so that an electric alternating voltage is applied to the measuring capacitor upon excitation of the LC oscillator. The through-opening is thus subjected to an alternating electrical field. The test subject is moved through the plate capacitor and is subjected to the alternating field. An electric output signal of the plate capacitor is detected. Dielectric properties of the test subject are determined from the output signal in an evaluation circuit. Changes in the parameters of the test subject such as mass per unit of length and/or material composition are determined from the dielectric properties. A capacitive yarn or sliver sensor is described for example in GB-638,365 A.
In order to enable the performance of precise measurements which are not influenced by external influences such as air temperature or air humidity, a compensation method is frequently applied. For this purpose, the apparatus comprises a reference capacitor in addition to the actual measuring capacitor. It can be formed by adding a third capacitor plate arranged parallel to the two measuring capacitor plates, with the three capacitor plates being switched together into one capacitive measuring circuit. Examples for measuring circuits and suitable evaluation circuits for their output signals can be found in the specifications EP-0,924,513 A1, WO-2006/105676 A1 and WO-2007/115416 A1.
A device for the capacitive quality control of textile threads is known from U.S. Pat. No. 4,843,879 A. It contains a double-capacitor arrangement with a measuring capacitor and a reference capacitor. The double-capacitor arrangement is built into an electric circuit. The circuit contains an oscillator for applying two alternating voltages with opposite phases to the two outer capacitor electrodes of the double-capacitor arrangement. A signal amplifier and a balancing capacitor are disposed in each branch between the oscillator and the respective electrode, with which the output signal of the double-capacitor arrangement can be balanced to the value of zero without any test subject.
WO-2006/069720 A2 discloses a measuring apparatus and a measuring method for the capacitive recognition of foreign bodies in a product, especially in tobacco, cotton or any other fiber product. A high-frequency generation device generates a high-frequency wave which is applied to the measuring capacitor. The voltage amplitude of the generated high-frequency wave is kept constant by means of a closed-loop control device.
An evaluation circuit for determining complex impedances is known from EP-0,922,963 A2. A voltage source generates two alternating voltage signals, of which one is applied to the impedance to be measured. The output signal of the impedance is combined with the other alternating voltage signal which is phase-shifted by 0° or 90° respectively in such a way that the real part and the imaginary part of the impedance can be determined from the respectively resulting output signals. The voltage source comprises a frequency generator for generating digital signals, a 1:2 frequency divider and a D-flip-flop for generating two alternating voltage signals phase-shifted by 90°. The phase shifting is maintained in the variation of the frequency.
JP-2002-005971 A shows an apparatus for capacitive differentiation of various fluids such as distilled water and tap water and for the detection of non-metallic foreign substances in fluids. For this purpose a circuit in which a sample is disposed is scanned in a broadband manner and the dispersion is determined. A direct digital synthesizer (DDS) is used as a signal generator for the scanning.
In the case of the capacitive sensors known from the state of the art, the measuring capacitor or measuring circuit is part of an LC oscillator. The measuring capacitor thus influences parameters of the alternating voltage applied to the same such as its frequency, phase and amplitude. On the other hand, an LC oscillator usually has a single resonant frequency, or at least a very limited number of discrete resonant frequencies, on which it can be operated. The resonant frequency and the phase can only be changed or balanced with a lot of effort.